The present invention relates to measuring the amount of relative movement between two mutually movable elements. The invention is suitable for use in numerous fields, particularly in the field of transportation systems wherein a vehicle (constituting the first element) moves on a guide track (constituting the second element). This application will be the main application referred to in the following, but the invention is also suitable for use in many other fields and notably in industrial installations in which an elongated product (constituting the second element) passes through an apparatus (constituting the first element), such as a rolling mill or a papermaking machine. In all these installations, there are systems for automatic control and monitoring which require measuring an amount of movement and/or a speed.
Many methods for measuring an amount of movement or speed are already known. In the railroad field, the most frequently used method consists in measuring the rotational speed of the wheels. That measuring method has a poor accuracy due to slipping. Furthermore, it requires periodical recalibration due to wheel wear. Speed measuring methods based on the Doppler effect have also been proposed, using a generator of microwaves which are back scattered by the track. That method is difficult to implement reliably and it does not give significant results at low speeds.
A measuring method has also been proposed (U.S. Pat. No. 4,162,509) including the steps of forming two successive images, each provided by a flash tube-photodiode array unit, the two units being at a fixed distance from each other; the flash tubes may be omitted if ambient light is available. A circuit adjusts the time interval between the two images so that the two images substantially correspond to the same area of the ground. A correlation between the two images that have been obtained is then made.
Last, document CH-A-531 178 discloses a method using two sources that continuously light a track with respect to which a vehicle provided with the measuring apparatus is moving. There is neither generation of an image of a common region, nor generation of an image of an interference array. The reflectivity of the lighted area is simply measured by a photoelectric converter. The output signals from the two converters are multiplied so as to obtain a cross-correlation function. As in the previously cited reference, the time delay between the samplings is adjusted. To obtain a significant result, it is essential that the time delay be very accurately adjusted responsive to the amount of movement of the vehicle relative to the track, and that represents a drawback.